For coating cars, industrial machines, steel furniture, interior and exterior decorations for constructions, household electric appliances, plastic goods and others, coating compositions consisting essentially of melamine resins such as alkydmelamines, acrylic melamines and the like have heretofore been used.
However, the coating compositions consisting essentially of such melamine resins are problematic in that they generate harmful formalin when cured. In addition, it is a more serious problem that the cured films produced by such melamine resins have poor acid resistance. These melamine resins are therefore often corroded, or etched, whitened or stained, by the acid rain caused by recent air pollution.
In order to solve this problem, coating compositions not containing melamine resins have been proposed (Japanese Patent Application Laid-Open (JP-A) Hei-3-287650 and Hei-2-45577). In those coating compositions, formed are ester bonds that act as crosslinking points, through reaction of an acid group with an epoxy group. Therefore, as compared with coating compositions consisting essentially of melamine resins, those not containing melamine resins have the advantage of forming coating films with good acid resistance and weather resistance.
Apart from the above, other studies have been made for using blends of hydrolyzable silyl group-having vinylic copolymers and acrylic polyols, or copolymers of hydrolyzable silyl group-having vinyl monomers and alcoholic hydroxyl group-having vinylic monomers in coating compositions (see JP-A Sho-63-132977).
The coating compositions comprising blends of hydrolyzable silyl group-having vinylic copolymers and acrylic polyols, or copolymers of hydrolyzable silyl group-having vinyl monomers and alcoholic hydroxyl group-having vinylic monomers are characterized in that they cure through stable siloxane or siloxy bonding to be formed by the reaction of the hydrolyzable silyl group and the alcoholic hydroxyl group, and, therefore, as compared with coating compositions consisting essentially of melamine resins such as acrylic melamines, alkydmelamines and the like, they form coating films having better acid resistance and weather resistance.
Generally, any of two types of metallic color finishing and solid color finishing are employed for finishing car bodies by top coating.
For the metallic color finishing 2-coat 1-bake method of heating and curing a coating composition, is generally employed coating a substrate with a metallic base coat followed by overcoating it with a clear coat of an acrylic melamine resin composition in an wet-on-wet system.
For the other solid color finishing, generally employed is a 1-coat 1-bake method of using an alkydmelamine resin composition, in which the composition is heated and cured to form top coating. For recent severe requirements for the properties of coating films, including, for example, finish appearance, weather resistance, acid resistance, scratch resistance and stain resistance thereof, a method of overcoating a solid color-finished substrate with a clear coat of an acrylic melamine resin composition or the like has been proposed.
The present invention has been made in consideration of the current situation noted above, and its object is to provide a curable composition for top coating, which has good thermal curability to form good coating films having well-balanced physical properties including good acid resistance, scratch resistance, weather resistance, outward appearance and recoating adhesiveness, and additionally having extremely excellent stain resistance, and also to provide articles coated with the composition.